This application is based on application No. Hei. 11-2364 filed in the Japanese Patent Office on Jan. 7, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a carbon-based active material for a rechargeable lithium battery, the rechargeable lithium battery and a method of preparing the carbon-based active material.
(b) Description of the Related Art
In recent years, the development of miniaturized portable electronics provokes needs for a rechargeable battery having a high capacity as well as a light weight. From the viewpoint of the capacity improvement per unit weight, the rechargeable lithium battery is preferably adopted because lithium has a high standard potential as well as a low electrochemical equivalent weight.
Metallic lithium has been used for the negative active material in a rechargeable lithium battery. However, during charging, lithium is electroplated onto the metallic lithium electrode, it deposits to form a dendric layer that increases the reactivity of lithium.
The problem of lithium has been addressed by replacing the lithium metal with carbon-based materials. Owing to the use of the carbon-based materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as the reasonable shelf life.
Among carbon-based material, graphite-based material has a high charge and discharge efficiency and voltage flatness. Graphite-based material is produced by heat-treating petroleum pitch or polymer resin at 2,000 to 3,000xc2x0 C. The resulting graphite-based material has hydroxyl groups on its surface, which react with an electrolyte during charge and cause decreases in a charge and discharge efficiency.
It is an object of the present invention to provide a carbon-based active material for a rechargeable lithium battery, which can increase charge and discharge efficiency.
It is another object to provide a rechargeable lithium battery having high charge and discharge efficiency.
It is still another object to provide a method of preparing the carbon-based active material.
These and other objects may be achieved by a carbon-based active material for a rechargeable lithium battery produced by heat-treating a carbon-based material under a fluorine atmosphere. The fluorine atmosphere serves to remove hydroxyl groups on the surface without the generation of Cxe2x80x94F covalent bonds. Accordingly, the carbon-based active material of the present invention has no hydroxyl groups or Cxe2x80x94F covalent bonds on the surface.
In order to achieve these and other objects and others, the present invention provides a rechargeable lithium battery including a positive active material and a negative carbon-based active material. The carbon-based active material has no hydroxyl groups and Cxe2x80x94F covalent bonds and produced by heat-treating a carbon-based material under a fluorine atmosphere. The positive active material is a material into or from which lithium ions are reversibly intercalated or deintercalated. The carbon-based active material is a material into or from which lithium ions are intercalated or deintercalated.
The present invention further includes a method of preparing a carbon-based active material for a rechargeable lithium battery. In the method, a carbon-based material is heat-treated under a fluorine atmosphere to remove hydroxyl group from a surface thereof without the generation of Cxe2x80x94F bond. The heat-treating step preferably includes the steps of injecting the carbon-based material into a reactor, and heating the reactor by the suitable temperature under vacuum. Next, gaseous fluorine is charged into the reactor and the carbon-based material is heat-treated for a suitable time. The gaseous fluorine preferably includes fluorine gas of at least 99.7% purity. The pressure of the gaseous fluorine is preferably 10 to 100 kPa. The second heat-treating step is preferably performed at 80 to 150xc2x0 C. for 15 to 30 minutes.